Silicon carbide in the form of whiskers with a high aspect ratio (of length to diameter) has been proposed for use as a reinforcement in aluminum metal to increase its modulus of elasticity, and greatly enhance its utility for the most critical aerospace applications. It has been reported that an addition of 25 volume % of SiC powder, of which about 20% was in the form of whiskers, increased the Young's modulus of the standard aircraft alloy 2024-T4 by approximately 100%, and the ultimate tensile strength by approximately 50%.
Silicon carbide occurs in two crystal forms, a .beta. cubic form and an .alpha. form in hexagonal and rhombohedral classes in a number of polytypes. Temperatures higher than 1800.degree. C. favor the formation of the .alpha. form, and temperatures between 1200.degree. to 1800.degree. C. favor the formation of the .beta. form. The .alpha. type is preferred as an abrasive, while the .beta. form has been less important.
SiC whiskers have been made by the method disclosed by Cutler in U.S. Pat. No. 3,754,076, (1973); by Adamsky & Merz, Z. Krist 111, 350-361 (1959); and by Hamilton, J. Appl. Phys 31, 112-116 (1960); see Kirk-Othmer Enc. Chem. Tech. Vol. 4, 2nd Ed. Wiley & Sons, N.Y., 1964, (117-118). Kirchner, U.S. Pat. No. 2,018,133 (1935) disclosed the use of buckwheat hulls. Japanese Application No. 52/113,300 by Tokai Carbon, Oct. 3, 1978, discloses heating rice chaff in a high frequency induction furnace to form .alpha.-SiC whiskers.
Considerable scientific study of whiskers has been done, and their properties have been found to be of great utility in reinforcing low modulus solids, but commercial development has been limited due to the extremely high cost of production. Typically they are produced in gas-phase reactions with low yields in expensive processes, which has limited availability.
One of the problems unsolved until the present invention was that of obtaining a reasonable yield of whiskers. When attempting to manufacture whiskers, about 20-25% of the SiC product obtained is the maximum obtained in the whisker form, the rest being irregularly shaped particles of little value as reinforcement.
It is difficult to separate the whiskers from the powder due to their small size. Mechanical screening is slow and imperfect, while air classification has so far been unsuccessful. Consequently, it is imperative to have the whiskers in high yield if the material is ever to become a large scale commercial product.